Generally, multilamp photoflash units may be characterized as either cube or linear array configurations. Cube unit configurations are illustrated by U.S. Pat. No. 3,730,669, referred to as magicube. A linear array configuration is depicted in U.S. Pat. No. 3,857,667, referred to as a flashbar, and planar array configurations are shown in U.S. Pat. Nos. 3,894,226 and 4,017,728, referred to as a flip flash.
Significantly, all of the above-mentioned multilamp photoflash units employ self-contained reflectors of a substantially parabolic design. An example is shown in U.S. Pat. No. 3,609,332. As is well known, a point source located at the focal point of a parabolic reflector has a high forward reflecting capability and provides a light beam which is substantially parallel and of a width equal to the reflector opening. However, a flashlamp is not a point source but rather a relatively large light source. As a result, the light is not reflected as a parallel light beam but rather is dispensed in a manner such that the light intensity is greater at a center region of a photographic zone than at the zones surrounding the center zone. Thus, the center zone of a photographic area tends to exhibit overexposure characteristics while the surrounding zones tend to be normal or underexposed.
As known in the art, the method of measuring light distribution across the photographic zone is by comparing the light output in the center 5.degree. to the average output of a 5.degree. zone between 15.degree. and 20.degree. inside the perimeter of the picture area. To our knowledge, prior art lamp reflector modules have provided a distribution between 1.6:1 and 2:1. This means that the center has nearly twice the exposure of the edge; this is nearly a full f-stop setting on the camera lens.
Ideally, a distribution ratio of 1:1 is desired; however, even a bare (unreflected) lamp will provide a distribution of about 1.3:1 because of the cosine law of light distribution. Elimination of a focusing reflector, of course, results in a significant loss of light output. Hence, in practice, a lamp-reflector system providing a 1.3:1 ratio of light distribution would be a significant improvement in light uniformity while retaining a degree of output enhancement.
A further consideration in the improvement of photoflash unit design is the provision of a compact reflector-lamp module to facilitate closer lamp spacing for a reduced package volume. U.S. Pat. No. 3,860,809 describes a lamp-reflector combination wherein the ratio of the width of the reflector opening-to-lamp envelope diameter is from about 1.2 to 2. Further the reflector is described as having an arcuate rear wall portion that is in nested relationship with the lamp envelope and has substantially flat outwardly diverging sidewall segments. U.S. Pat. No. 3,991,308 shows a reflector having a cylindrical central portion and flat side portions. U.S. Pat. No. 3,267,272 shows a reflector with a substantially V-shaped cross-section.
We have found that the arcuate, cylindrical and V-shaped rear portions of the reflector cavities are comparatively disadvantageous with respect to light output in very reduced (compact) ratios of reflector opening-to-lamp diameter. More specifically, a planar rear segment has been found to provide a gain in zonal lumen output. U.S. Pat. No. 3,508,040 shows a reflector with a planar back portion and a pair of single planar side portions.
The simplest and least costly method of producing reflectors is by thermo-vacuum-forming of thin, thermoplastic film. With reduced ratios of reflector opening-to-lamp diameter, however, excessive thinning of the rear portion of the reflector results. The aforementioned planar rear section design with single flat sides exagerates the problem even further; for example, we have observed plastic thinning at the corners of the cavity walls whereby the minimum thickness is less than 10% of the maximum thickness. The smaller the reflector opening-to-lamp diameter or, conversely, the larger the lamp diameter-to-reflector opening, the greater the plastic thinning.